With the increased need for higher beam quality and output powers in industrial applications, laser manufacturers will turn to windows constructed of synthetic diamond.
At issue are the material properties of the substances, said Ricardo S. Sussmann, the former head of research and development at DeBeers Industrial Diamonds (UK) Ltd. of Ascot, UK. ZnSe has a very low coefficient of absorption at 10.6 µm but, as laser powers increase, windows made of it suffer from thermal lensing, gradients in the refractive index that distort the beam.

Although synthetic diamond has a coefficient of absorption 100 times higher than ZnSe at this wavelength, its refractive index is less dependent on temperature. Moreover, diamond conducts heat 120 times better than ZnSe and is seven times as strong. The net result is that at 5 kW a diamond window displays 200 times less beam distortion than ZnSe.

Despite the benefits, there is a substantial difference in price, Sussman cautioned. Although the chemical vapor deposition growth technique and optical fabrication approaches have been mastered, they remain expensive. A diamond window may cost up to five times as much as the same window in ZnSe. "If you can use zinc selenide, you will use zinc selenide," he said.

Sussmann, who is now a private technical consultant and part-time senior research fellow at King's College, said that after 12 years diamond growth technology is mature but that only a few companies have established full-scale operations. Nevertheless, high-power CO2 laser manufacturers are offering systems with the windows, and demand is expected to increase with the next generation of lasers, where the power-to-beam-diameter ratios will exceed 1 kW/mm.

Symmetrical Beam

Rofin-Sinar Laser GmbH of Hamburg, Germany, for example, has already incorporated synthetic diamond windows into its series of diffusion-cooled CO2 slab lasers. The 1- to 3.5-kW systems, which are designed for cutting, welding and surface treatment applications, owe their highly symmetrical beams to the properties of the windows. "That laser could not work with anything but diamond," Sussmann said.